This project would continue the research funded by grant MH41704 on human concepts, their representation in the mind, and their relation to word meaning. The work investigates two main issues. The first issue, category learning, involves the coordination of empirical experience with category members as well as prior expectations and beliefs about what kinds of categories are sensible and likely. Although much research has focused on one or the other of these issues, the present research addresses their interaction. In particular, the research follows up the unexpected finding of the lab's previous work that when knowledge relates some aspects of a concept, those aspects are preferentially learned, but other aspects are not learned less well (than a control group). A series of experiments on concept acquisition explores the possible interactions of empirical and knowledge-based learning to explain this effect. A second series of experiments investigates how prior knowledge influences the construction of properties included in a concept representation. Although we tend to think of an object's properties as given to us by perception, in fact, people notice and encode different aspects of an object depending on their prior categorization history. The proposed experiments manipulate causal knowledge of an entity in order to discover whether such knowledge can influence the unitization of experience, thereby altering the perception of a part. The second main issue is the relation between word meaning and concepts, as revealed through the phenomenon of polysemy -- the fact that many content words have numerous related meanings or senses. A series of studies is proposed that examines how people can understand a word when it is used in a novel sense, which occurs frequently in normal language use. Unlike previous work on this topic, the studies will use a realistic linguistic setting and will obtain online measures of comprehension to test its hypotheses. This research investigates some of our most basic thought processes -- how we perceive, identify, think about, and talk about the objects and events in our world. As these processes are part of the basic building blocks of intelligent behavior, they are critical to understanding both normal and abnormal behavior. A complete understanding of concepts is necessary to understanding the breakdown of thought in brain damage and mental illness.